/Engine/Source/Runtime/GeometryCore/Public/Util/IndexUtil.h
Find ordered edge [VertexID1,VertexID2] in TriangleVerts and then return the index of the remaining third vertex
index of third vertex, or InvalidID if not found
namespace IndexUtil { template<typename T, typename Vec> int IndexUtil::FindTriOtherIndex ( T VertexID1, T VertexID2, const Vec & TriangleVerts ) }